Conventionally, before emitting so-called “industrial waste gases”, such as exhaust gases of boilers in factories and smoke from power generating plants, into the atmosphere, air purification processing is performed to remove various types of fine particulates contained in the industrial waste gases, such as mist and dust containing, for example, powders containing oil, moisture, and/or the like, which can pollutes the atmosphere. Direct emission of the industrial waste gases containing the fine particulate into the atmosphere significantly affects the global environment, so that there are cases where it is obligatory to perform collection thereof by standards of a national or local governments. Dust collection is one of several purification techniques used to collect fine particles contained in polluted air.
The dust collection principle of electrostatic dust precipitators is such that electric charges are supplied to the fine particulate through corona discharges generated from discharge electrodes, and coulomb forces are used to electrostatically attract the charged fine particulate onto collector electrodes which are opposed electrodes, whereby the fine particulate is collected. An electrostatic precipitator has significant advantages, such as: 1) the pressure loss is small; 2) a large amount of gases can be processed; and 3) the collection efficiency is high. Therefore, electrostatic precipitators are commonly used in such environments as factories and power generating plants which emit a large amount of polluted air.
Generally, the construction of an electrostatic precipitator includes discharge electrodes each formed into a shape having a large surface curvature, such as a needle or wire material, for generating corona discharges for supplying electric charges to the fine particulate; collector electrodes, as opposed electrodes, each formed into a flat plate for collecting the charged fine particulate; a rectifier unit for rectifying the flow of gas streams in the electrostatic dust precipitator; a dust removal device for separating deposited fine particulate from the collector electrodes; a hopper for collecting the separated fine particulate; and a power system and electric charge control device to cause corona discharges. In using the dust removal unit of a electrostatic precipitator, the collector electrodes are rapped by a hammer or the like to dislodge collected fine particulate therefrom and collect the fine particulate in a collection unit, such as a hopper, provided in a lower portion. When a large amount of the fine particulate is collected onto the collector electrodes, the coulomb force for attracting the charged fine particulate may be reduced, thereby reducing the collection efficiency. Therefore, dust removal units are important in order to remove a build up of the fine particulate from the collector plates.
However, the cleaning of the collection plates of the prior art systems presents a serious problem inasmuch as a substantial amount of reentrainment of the dust occurs. Because of the proximity of the plates to the gas flow channel, some of the dust particles dislodged during the rapping process are reintroduced into the gas stream. The reentrained particles must then be recharged and again collected for effective removal from the gas stream.
Prior attempts to increase the efficiency of the prior art systems have included the use of additional collection plates in the gas flow channel to increase the surface area available for deposition of the dust. Although producing some increase in efficiency and length of operating times between cleanings, this approach results in significant increases in the overall size and complexity of the precipitation systems and is therefore not entirely satisfactory.
Precipitators have also been proposed in which the particle collection surfaces are in the form of endless belts that continuously move through the collection zone. For example, in the U.S. Pat. No. 2,579,440 to Palmer, the contents of which are hereby incorporated by reference, dust particles are collected on a moving belt that has been charged to a polarity opposite to that imparted to the dust particles and are removed from the belt surfaces at a point remote from the gas stream. Such apparatus is, however, subject to the disadvantage that the charges applied to the surfaces of the belts are susceptible of being pulled off the belts by the charged dust particles. When this occurs, the particular dust particles affected are not removed from the gas stream and, in addition, the precipitation field is weakened because the charge carried by the belts is reduced.
It is an object of this invention to enhance electrostatic precipitation efficiency by combining an improved collection surface in which the electrostatic field forces acting on the deposited particles are strong enough throughout the collection surface to capture and retain the particles, and an improved dust collection system which reduces the incidence of particle reentrainment.